1. Field of the Invention
The present invention relates to a focus detection apparatus for use with an optical apparatus such as a camera and, more particularly, to a TTL type focus detection apparatus which detects the focusing state by an imaging light beam from a photographic lens in a single reflex camera.
2. Description of the Prior Art
Various types of focus detection apparatuses which may be used for single reflex cameras have been conventionally proposed. A focus detection method utilizing such as apparatus is disclosed, for example, in U.S. Pat. No. 3,875,401 (issued Apr. 1, 1975). According to this method, an imaging means is disposed at the rear side of the imaging plane of the photographic lens so as to reform the first object image formed on the imaging plane as a plurality of second object images. The focusing state of the phototaking lens is detected by detecting the relative positional relationship between the plurality of second object images.
FIG. 1 is a perspective view schematically showing a conventional focusing detection apparatus adopting such a focus detection method. Referring to FIG. 1, a field mask 13 having a slit-like opening 13a is disposed in the vicinity of an imaging plane 12 of a photo-taking lens 11. A field lens 14 is disposed immediately behind the field mask 13.
The object image, which is formed on the field mask 13 by the photo-taking lens 11, is reformd as the two second object images. The imaging lenses 15a and 15b form the two second object images on the basis of light beams L1 and L2 emerging from different positions of the exit pupil of the photo-taking lens 11 by means of the field lens 14. Light-receiving means 17a and 17b are disposed in the vicinity of an imaging plane 16 of the imaging lenses 15a and 15b. Each of the light-receiving means 17a and 17b comprises a plurality of photoelectric conversion elements (generally, self-scanning type Charge Coupled Devices).
Method for determining the relative positional relationship between the two second object images formed on the light-receiving surface of the light-receiving means 17a and 17b, respectively, in accordance with the light quantity distribution of each second object image, include a method for calculating a correlation amount in real space and a method for calculating a phase difference in frequency space.
The operation method in real space will first be described. When an nth output, that is, a sampling value from a plurality of photoelectric conversion elements of each of the light-receiving means 17a and 17b is designated as a.sub.n or b.sub.n, the respective second object images formed on the light-receiving surface of the light-receiving means 17a and 17b are only shifted along the direction perpendicular to an optical axis L in accordance with the degree of de-focusing of the photo-taking lens 11, and have the same shape. Accordingly, we obtain: EQU b.sub.n =a.sub.n-k ( 1)
where k is the relative displacement amount between the two second object images, which changes in accordance with the degree of de-focusing of the photo-taking lens 11. That is, K=0 when the photo-taking lens 11 is in focus. The purpose of the focusing state discrimination algorithm is basically the determination of the relative displacement amount k, and this may be achieved by introducing a shift-operation into the real space operation method. For example, a correlation amount is given as: ##EQU1## where j is the shift amount. When j=K, V(j)=0 from equation (1) above. Accordingly, if V(j) given by equation (2) is calculated for various values of j, the focusing state of the photo-taking lens 11 can be discriminated.
Since the sampling values of the light quantity distribution of each second object image from the photoelectric conversion elements are discrete, the relative displacement amount k between the two second object images may not necessarily be an integer multiple of the pixel pitch which is determined by the size of the photoelectric conversion elements. Data correction must be performed so as to determine the relative displacement amount k in terms of fractions of the pixel pitch. A complicated relation is frequently used as a correlation amount suitable for such correction. For example, U.S. Pat. No. 4,333,007 (issued June 1, 1982) discloses the operation method which uses the equation: ##EQU2##
The operation method in frequency space will now be described. The Fourier transform of a.sub.n at a spatial frequency m is given by: ##EQU3## where N is the number of pixels of each of the light-receiving means 17a and 17b and i is the imaginary number unit .sqroot.-1. Using this Fourier transform, the Fourier transform Bm of b.sub.n at the spatial frequency m can be obtained from equation (1) as: ##EQU4## That is, when the two second object images change position relative to each other, their Fourier transforms differ from each other by a phase factor exp (-i2.pi.mk/N). Since the phase difference 2.pi.mk/N is proportional to each of the spatial frequency m and the relative displacement amount k, the focusing state of the photo-taking lens 11 can be discriminated by calculating the Fourier transforms of the respective images and comparing their phases. A method for electrically calculating Fourier transforms and for obtaining the relative displacement amount k of the second object images is disclosed, for example, in U.S. Pat. No. 4,264,810 (issued Apr. 28, 1981).
However, in any of such methods, the light quantity distribution of the second object images must be measured at a plurality of points. Accordingly, when the light-receiving means 17a and 17b comprise self-scanning type charge coupled devices (CCDs) each having a number of pixels, the following problems are encountered.
(i) Since a CCD has a different dark current and sensitivity for each pixel, good light quantity distribution data is hard to obtain with a CCD having a number of pixels. In the apparatus shown in FIG. 1, discrimination of the focusing state becomes unreliable with either of the methods described above. From a different point of view, a CCD which has a number of pixels and which also provides good data has a low yield and a high manufacturing cost. Accordingly, a conventional apparatus as shown in FIG. 1 becomes expensive if such a CCD is adopted.
(ii) Each of the methods described above requires a large number of operation steps and an expensive electrical processing unit. When the number of pixels of a CCD is designated by N, and the method of equation (3) is adopted in the apparatus shown in FIG. 1, N.sup.2 operations and a 2N-byte random access memory are required. When the method of equation (5) is adopted in the apparatus shown in FIG. 1, a complex electrical circuit for calculating Fourier transforms is required.
(iii) A large amount of operation process impairs the real time discrimination of the focusing state. Extremely expensive hardware must be used so as to achieve real time processing.
(iv) In the apparatus shown in FIG. 1, in order to perform fine sampling, the pixel size of the light-receiving means must be decreased, resulting in an increase in the accumulating time of the light-receiving means. Then, the operation of the focusing apparatus of a camera of a general user becomes unstable.